# Evaluate constants in symbolic expression?

Hello. If I have a symbolic expression where all of the variables are known, I can use `numerical_approx`

to evaluate it:

```
var('a b')
x = a / b
x(a=1, b=3)
> 1/3
x(a=1, b=3).numerical_approx()
> 0.333333333333333
```

However, if there is an unknown variable, `numerical_approx`

fails:

```
var('a b c')
x = a / b * c
x(a=1, b=3)
> 1/3*c
x(a=1, b=3).numerical_approx()
> TypeError: cannot evaluate symbolic expression numerically
```

Is there a way to approximate the values of all constants / coefficients in a symbolic expression? Essentially, something like

```
var('a b c')
x = a / b * c
x(a=1, b=3).numerical_approx_constants()
> 0.33333333333*c
```

Thank you in advance.

Using https://ask.sagemath.org/question/460... you can do

`SubstituteNumericalApprox()(x(a=1, b=3))`

.As another option, don't substitute constants by integers, but by floats: